In the manufacture of printed circuit boards, a photoresist is used to transfer the outline of the circuit into the copper surface of the board. The name photoresist defines the dual functioning nature of this material. First it is a photopolymer whose chemical properties are changed by exposure to ultraviolet radiation. That exposure is done selectively through a mask outlining the circuit being defined. The dual functioning comes into play after developing the photopolymer, where the soft unwanted areas are washed off the copper surface. What remains is a protective covering of hardened polymer only in those areas outlined by the exposure mask. In one application this protective covering resists the etching process so that only the copper left unprotected is etched away. When the resist is finally removed, the protected copper circuit lines underneath become the electrical conductors of the circuit board.
One real measure of the evolution of printed circuit board technology is the width of the copper circuit lines and the spacing between them. As the component density and circuits per square inch increase, the width of the circuit lines and the spaces between them must decrease. The current state of the art is 10 mil lines with 10 mil spaces. This geometry is ultimately determined by the process technology that allows the reliable fabrication of circuit boards within tolerances acceptable to the industry. In normal production a 10 mil wide circuit line can be controlled to within plus or minus 1 mil. If this line is spaced 10 mils from an adjacent line that may also vary by only 1 mil, there is little chance of having broken lines or short circuiting between lines. If, however, that line spacing geometry is reduced to 1 mil lines and spaces, the previous tolerance is unacceptable and the process technology must be advanced to achieve and maintain a tighter tolerance.
Two methods are commonly used for applying photoresist to the copper surface of a circuit board. One of these is coating and the other is lamination. In coating, a fluid containing the photopolymer dissolved in solvent is applied to the copper surface in a thin uniform layer. The solvent is evaporated away and a uniform film of photoresist is deposited onto the copper surface. In lamination, a previously coated and dried film of photoresist on a carrier web is bonded to the copper surface using heat and pressure, after which the carrier web is stripped away.
Most of the circuit boards produced today use the dry film method for two primary reasons. First there is no solvent fluid to cause safety, personnel, environmental or disposal problems. Secondly, there is no liquid photoresist to get inside the dill-through holes to contaminate them and jeopardize the integrity of the plated through connections. These two advantages of dry film over coating are substantial but they are obtained at a price. One price is economic as dry film is about three times as costly per square foot as a coated photoresist. Additionally, dry film is a wasteful process, with the resist overhanging the board edges, for example, being totally useless. Dry film also requires the maintenance of substantial and varied inventories in order to cover the various etching and plating operations. Dry films are also susceptible to loss of resolution through overexposure and etchant leaching beneath the resist.
The other price, far more costly, is technological. Dry film has not been produced reliably below a thickness of one mil. In order to reduce the line spacing geometry so that circuit density can be significantly increased, it is necessary to reduce the thickness of the photoresist to around 0.1 to 0.2 mils.
Accordingly, as the demand for finer resolution has grown, greater consideration has been given to liquid photoimageable primary resists. The available application techniques for such liquids have also, however, exhibited certain disadvantages. Spin coating is laborious and does not allow for high volume coating. Dip coating does not assure even coating thicknesses. Roller coating is a slower process which must accommodate potential contamination of the rollers. Electrostatic spray coating involves a more complex approach with the potential for waste of the photoresist spray. Finally, electrocoating requires the application of plating technology with its greater complexity, higher cost and required close bath monitoring.
Curtain coating approaches wherein the circuit boards are conveyed beneath a curtain of photoresist have been successfully adopted for use in producing solder masks on printed circuit boards. Such a curtain coating approach and the applicable parameters have been described in U.S. Pat. No. 4,230,793. However, in view of the need to apply very thin coatings on thin inner layer core and to coat the second side of the board without damaging the first coated side, curtain coating has not been suggested for primary photoimaging. In fact, it has been suggested that only minimum deposited film thicknesses of about 50 microns can be achieved while still retaining a stable resin curtain. [see ZEV-Leiterplatten, pp. 20-24 (11/90)]. Alternatively, somewhat thinner film deposition requires rapid board transport beneath the curtain and/or substantial viscosity reduction, neither approach ensuring proper deposition and/or curtain stability.
An article by Dr. N. Ivory in PC Fab. pp. 30-38 (April 1990) also suggests the possibility of the curtain coating approach for primary imaging of circuit boards. This discussion, however, makes reference to the above noted undesirable viscosity reduction and increased substrate feed rate in order to utilize such a technique. Two additional problems are identified. Thus, difficulties are noted in coating thin innerlayer laminates due to bending and buckling phenomena. Carrier frames are therefore required in such circumstances. Protection from etchant attack of the plated through holes in double-sided and multilayer boards is also a problem whose solution requires costly additional processing.